Biological Classification

Classification of Organisms

• Aristotle attempted to establish a scientific basis for classification, using simple morphological characters to group plants into trees, shrubs, and herbs, and animals into two groups based on the presence or absence of red blood.

Two Kingdom System of Classification

• The Two Kingdom system, developed in Linnaeus' time, categorized all living organisms into either Plantae (plants) or Animalia (animals) kingdoms.
• This system did not distinguish between eukaryotes and prokaryotes, unicellular and multicellular organisms, and photosynthetic and non-photosynthetic organisms.

Limitations of the Two Kingdom System

• The Two Kingdom system was found to be inadequate, as it did not account for a large number of organisms that did not fit into either category.
• There was a need to include other characteristics, such as cell structure, nature of wall, mode of nutrition, habitat, methods of reproduction, and evolutionary relationships, in the classification system.

Five Kingdom Classification

• R.H. Whittaker proposed a Five Kingdom Classification system in 1969, which included the kingdoms Monera, Protista, Fungi, Plantae, and Animalia.
• The main criteria for classification used by Whittaker included cell structure, body organization, mode of nutrition, reproduction, and phylogenetic relationships.

Characteristics of the Five Kingdoms

• Table 2.1 provides a comparative account of the different characteristics of the five kingdoms.

Evolution of Classification Systems

• The understanding of what groups/organisms belong to each kingdom has changed over time, and different scientists have proposed different classification systems.
• The three-domain system has also been proposed, which divides the Kingdom Monera into two domains and leaves the remaining eukaryotic kingdoms in the third domain.

Issues with Earlier Classification Systems

• Earlier classification systems grouped together organisms with cell walls, including bacteria, blue-green algae, fungi, mosses, ferns, gymnosperms, and angiosperms, under the 'Plants' kingdom.
• This classification system did not differentiate between prokaryotic and eukaryotic organisms, unicellular and multicellular organisms, and heterotrophic and autotrophic organisms.

Changes in Classification Systems

• The classification system changed to include characteristics such as cell structure, mode of nutrition, and phylogenetic relationships, leading to the separation of fungi into a separate kingdom and the grouping of unicellular eukaryotic organisms into Kingdom Protista.